1. Field of the Invention
The present invention relates to a circuit for monitoring an audio output signal, and, more particularly, to a circuit for monitoring DC Offset in an audio output signal.
2. Description of the Related Art
The presence of Output DC Offset is a fault condition which can occur in audio power amplifiers. Typical loudspeakers employing a voice coil-magnet combination with a paper cone are quite susceptible to damage by DC Offset, so this potential condition should be avoided if at all possible. Automotive power amplifiers often employ integral DC Offset detection, but this feature may test for this condition only at initial power-up (e.g., “Turn-On Diagnostics”). If a fault condition occurs after “turn-on,” the fault condition may remain undetected and component damage may occur.
The DC Offset detection may prevent smoke emission, ignition and other defects caused by a short-circuit of an audio power amplification IC input that drives a speaker. A DC offset may result due to a short-circuit 10 (FIG. 1) caused by an adhesion of burrs between a ground (GND) terminal 12 and an input signal terminal 14 which are neighboring or adjacent to each other in an audio power amplification integrated circuit (IC) 16. As a result of the short circuiting burr, the voltage at input signal terminal 14 becomes low, and a voltage difference develops between terminal 14 and terminal 18. With a standard internal voltage applied at node 19, the voltage difference becomes amplified by amplifier 20, and a DC voltage develops across terminals 22, 24. Terminal 24 may be the negative speaker output and terminal 22 may be the positive speaker output. Thus, the DC voltage on terminal 24 may be higher than the voltage on terminal 22. As a consequence of the DC offset, DC current flows into a speaker 26 and a voice coil part (not shown) in speaker 26 generates heat, which causes defects such as smoke emission and ignition.
This defect cannot be prevented by a protection function for an output short-circuit such as thermal shut down or over-current detection. It is impossible to detect the defect even using 4A over-current detection mounted in an IC. The most effective known way to prevent the defect by a design is to use DC-OFFSET detection function.
As described above, the prior art includes an audio power amplification IC with a DC-OFFSET detect function. However, integral DC Offset detection typically has technical limitations, and the presence of this feature within the power amplifier IC increases the cost of the device.